TRAINING MANUAL- PIPING

PIPE DESIGN

Uhde India Limited

DOC No. : 29040-PI-UFR-0006

Rev. : R0

Page : 1

CONTENTS

Page

0.0 Cover Sheet 1

1.0 General 2

2.0 Thickness Calculation as per ASME B 31.3 2 – 3

3.0 General Notes 3 – 6

4.0 Steps to determine Pipe Wall Thickness 6

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TRAINING MANUAL- PIPING

PIPE DESIGN

Uhde India Limited

DOC No. : 29040-PI-UFR-0006

Rev. : R0

Page : 2

1.0 GENERAL:

The following method is issued for metallic pipes to calculate the wall thickness. The followingparameter are to be known:

• Internal Design pressure.• Design temperature.• Pipe material specification.• Pipe size (O.D. or I.D.)• Applicable code (ASME B 31.3/ IBR/ Other)

2.0 THICKNESS CALCULATION AS PER ASME B 31.3:

The required thickness of straight sections of pipe as per ASME B 31.3 is given by

tm = t + c

where,

tm = Minimum required thickness including mechanical, corrosion and erosion allowances

t = Pressure thickness in order to sustain internal design pressure P

c = Sum of mechanical allowances (Thread or groove depth) plus corrosion and erosion allowances.

If ‘T’ is the Nominal pipe wall thickness then, T ≥ tm + Manufacturer’s negative tolerance.

As per code,

PD (Cl. 304.1.2) . . . . . . . . . . (Eq.1) 2(SE+PY)

where,

P = Internal design gauge pressure, psigD = Outside diameter of pipe, inch

S = Allowable stress value for the pipe material, psi

E = Quality factor (Longitudinal weld joint efficiency for pipe)

Y = Coefficient as per Table - I, valid for t < D/6 and for materials shown. The value of Y (dimensionless factor varying withtemperature) may be interpolated for intermediate temperatures.

d = Inside diameter of pipe.

T =

TRAINING MANUAL- PIPING

PIPE DESIGN

Uhde India Limited

DOC No. : 29040-PI-UFR-0006

Rev. : R0

Page : 3

TABLE - 1

Value of coefficient Y for t < D/6

TEMPERATURE ° F

MATERIALS 900 & 950 1000 1050 1100 1150 lower

Ferritic steels 0.4 0.5 0.7 0.7 0.7 0.7

Austenitic steels 0.4 0.4 0.4 0.4 0.5 0.7

Other ductile 0.4 0.4 0.4 0.4 0.4 0.4 Materials

Cast Iron 0.0 - - - - -

Following equation may also be used as per code in lieu of (Eq.1) for calculating pressurethickness ‘t’ to provide pressure integrity

t = P D (Cl. 304.1.2) 2 SE

OR SE – P (Lame’s Equation)t = D 1 -

2 SE + P

OR

t = P(d+2c) (where thickness is calculated based on inside diameter) 2 [(SE-P(1-Y)]

3.0 GENERAL NOTES :

• Limitation on diameter to thickness Ratio:

Formulae as given are applicable for t ≤ D/6, which in fact is the usual case.However, for t ≥ D/6 or for P / SE > 0.385, calculation of pressure design thickness formetallic straight pipe requires special consideration of factors such as theory of failure, effectof fatigue and thermal stress.

TRAINING MANUAL- PIPING

PIPE DESIGN

Uhde India Limited

DOC No. : 29040-PI-UFR-0006

Rev. : R0

Page : 4

• ‘S’ Allowable Stress Values:

Allowable stress values for different ASTM pipe materials at various temperature are listedunder Table A1 ASME B 31.3 (Appendix A)

e.g. Allowable stress for:

A53Gr.B at 200 °F = 20,000 psiA53Gr.B at 500 °F = 18,900 psiA106Gr.B at 600 °F = 17,300 psi

• ‘E’ weld joint efficiency (Quality factor):

Weld joint efficiency (Quality factors) for different ASTM pipe material specifications are listedunder Table A1B ASME B 31.3

e.g. Quality factors for:

A53 ERW = 0.85A53 Seamless = 1.00A312 Seamless pipe = 1.00A312 EFW double butt seam = 0.85A312 EFW single butt seam = 0.80

(For all seamless pipes ‘E’ value is 1.00)

• ‘C’ Sum of Mechanical, corrosion and erosion allowances.

Following are the usual allowances to be accounted.

1. Threads:

This is applicable if the pipes are threaded for making joints as is the case with galvanizedpiping. The nominal thread depth has to be accounted under such situation. Refer table314.2.1 “ Minimum Thickness of male threaded components”

2. Corrosion/erosion allowances:

These allowance depend upon the type of fluid handled and are indicated by the Processlicenser. These allowances vary from 1mm to 6mm, and in some cases even more. As agood engineering practice, it is advisable to consider minimum 1mm corrosion allowance forall other services where Process licenser has not specifically indicated any requirement. Thisalso takes care of external corrosion if any.

3. Bending Allowance:

If the pipes are to be used for making bends, then it may be necessary to increase thethickness ‘tm’ by a factor called bend thinning allowance. During bending the outer fibres getstretched and in order to maintain minimum wall thickness ‘tm’ at all point in a completedbend, one has to add allowance for thinning.

TRAINING MANUAL- PIPING

PIPE DESIGN

Uhde India Limited

DOC No. : 29040-PI-UFR-0006

Rev. : R0

Page : 5

Flattening of a bend, the difference between maximum and minimum diameters at any crosssection, shall not exceed 8% of nominal outside diameter for internal pressure.

Following Bend thinning factors are recommended (refer Table C-4.5 “ Pipe Thickness forbends” – Piping Handbook 7th Edition by M. L. Nayyor).

Radius of Pipe Bend Min. THK. recommended prior to bending

6D (nom. Dia) 1.06 tm5D 1.08 tm4D 1.14 tm3D 1.25 tm

• Manufacturer’s Negative Tolerance:

While specifying the pipe thickness for ordering, it is necessary to account for Manufacturer’snegative tolerance since we require minimum thickness ‘tm’ at all points after the pipes aremanufactured.

The tolerances depend upon the method of manufacturing pipes and these are given inrespective ASTM PIPE material specs. The negative tolerance on specified thickness is 12 1/2% for seamless pipes.

Thus for seamless pipes if ‘tm’ is the minimum thickness required then the nominal thicknessT should be equal or greater than tm / 0.875.

Similarly, for electric fusion welded steel pipes as per ASTM A672 the manufacturer’snegative tolerance is 0.01 inch (0.3mm). Hence for pipes conforming to A 672 nominalthickness T should be equal or greater than (tm + 0.01 inch)

• Schedule-Number Selection

After calculating ‘T’ the nominal wall thickness which is required for design conditions, onecan order the pipes provided the quantity is large enough for special rolling. Otherwise, as perANSI B36.10 for Carbon steel and ANSI B36.19 for stainless steel, Pipes are readily availablein various thickness specified by their schedule numbers

It is recommended to make use of these standard pipe thickness which are available.Schedule number selected should have nominal thickness equal or greater than thecalculated nominal thickness required for design condition.

• Structural Rigidity:

Some times we may arrive at a very low thickness which is adequate to sustain pressure -temperature conditions given but such pipes if used will not have enough structural rigidityand may get deformed even during handling. These pipes if used would require carefulhandling and closer supports.

TRAINING MANUAL- PIPING

PIPE DESIGN

Uhde India Limited

DOC No. : 29040-PI-UFR-0006

Rev. : R0

Page : 6Unless the material is very costly, we usually decide minimum thickness for various pipe sizesat the initial stage of the project on structural rigidity criteria.

Following minimum thickness excluding corrosion allowance are recommended.

Pipe Size Minimum Thickness

1/2” to 4” 0.06”5” to 12” 0.09”14” to 24” 0.12”

For Carbon Steel piping in refinery and petrochemical plants especially for process lines,following minimum schedules are usually adopted.

Pipe Size Schedule No.

1/2” to 2” Sch. 802 1/2” to 10” Sch. 4012” to 24” STD Wall

For utilities like water, air and general purpose services one can adopt lower minimumschedules indicated below:

Pipe Size Schedule No.

1/2” to 1 1/2” Sch.40Sch.80 (If 5D bends are to be made at site)

2” to 6” Sch.408” to 24” Sch.20

4. STEPS TO DETERMINE PIPE WALL THICKNESS:

• Using the minimum inside diameter determined from the fluid flow evaluation, select thenext larger standard nominal size or outside diameter (OD) pipe from the listings providedin ASME B 36.10M for Standard Wrought Steel Pipe or B 36.19M for Stainless Steel Pipe.

• Based upon the Fluid and Service select a suitable piping material and if necessarydetermine the corrosion, erosion, joining or mechanical strength allowances.

• Using equations provided in the design Code, calculate the required minimum wallthickness to provide for pressure integrity and allowances.

• Refer to ASME B 36.10M or ASME B 36.19M to select an appropriate nominal wallthickness or Schedule.

• Confirm that the standard manufacturing tolerance will not reduce the minimumcalculated wall thickness or selected nominal wall thickness in the steps above.

• Confirm that the inside diameter of the pipe based on the selected nominal wall thicknessis compatible with the required inside diameter based on the fluid flow evaluation.